The present invention relates to a method and system for the production of resin coated fibers.
Composite materials are enjoying a steady and significant increase in popularity across many different products and industries. Fiber-reinforced resin composites have gained an increasing market share for high performance parts used in various industries, as for example automotive and aircraft components. Generally, such composites are formed from a "prepreg" that includes carbon or glass fibers impregnated with a polymeric resin. The impregnated fibers are wound or layered against rigid mold surfaces and cured into structural parts. Sometimes the fibers are woven or braided to enhance various properties of the composite.
A "towpreg" is one type of composite prepreg which is formed by impregnating long continuous bundles of reinforcing fibers called "tows." Tows of carbon or glass filaments are commercially available and may vary widely in the number of filaments per tow. Many matrix resins are also available; however, two kinds of matrix resin systems are of particular interest: thermoset and thermoplastic polymers.
A thermoset resin usually reacts in the presence of heat or a catalyst to produce a 3-dimensional structure that cannot be reshaped. Epoxies and certain polyimides are examples of thermosetting resins used in composite technology. Generally, thermoset materials have a relatively low viscosity during fiber impregnation.
In contrast, thermoplastic materials often have a significantly higher viscosity, typically 10.sup.3 -10.sup.7 times greater than thermoset resins. This relatively higher viscosity makes fiber impregnation with thermoplastic resins more difficult compared to typical thermoset resins. However, unlike thermoset resins, a thermoplastic resin retains its two-dimensional structure in the presence of heat and pressure. As a result, thermoplastic resins can be softened or melted many times, thus allowing for reshaping. Poly(aryl) ether ketone ketone (PEKK), Polyether-etherketone (PEEK), and ultra high molecular weight polyethylene (UHMWPE) are examples of high performance thermoplastics used in forming composites. Thermoplastic matrix composites are drawing increased interest, in part due to their excellent toughness, impact and chemical resistance, and potential for shorter processing times when compared with their thermoset counterparts. Unfortunately, the high melt viscosities of thermoplastic polymers hinder the uniform impregnation of the composite fiber with resin during component manufacture. For thermoplastic composite materials to expand beyond their current high performance and niche markets, cost-effective processes must be developed that rapidly and uniformly impregnate fibers with resin and permit greater control over resin content.
Fiber towpregs can be produced by a number of impregnation methods including hot melt, solution, emulsion, slurry, surface polymerization, fiber commingling, film interleaving, and dry powder techniques. Among these techniques, dry powder processing is emerging as an attractive method for achieving a low-cost manufacturing process. Dry powder processes deposit polymeric resin particles directly on the fibers and then fix the powder particles onto the fibers to form a towpreg. The significant advantages of dry powder techniques are the elimination of solvents, the reduction in stress on the fibers, and the improved reclamation of unused resin powder. Avoidance of solvents and recovery of unused powder reduces environmental hazards and provides significant economic advantages as well. Various dry powder processes are described in U.S. Pat. Nos. 3,742,106 to Price, 4,614,678 to Ganga, 5,057,338 to Baucom et al., 5,094,883 to Muzzy et al., 5,123,373 to Iyer et al., and 5,370,911 to Throne et al.
One dry powder process for impregnating glass or carbon fibers uses a spray gun to direct polymer powder particles entrained in an airstream onto a continuous fiber tow. Prior to discharge from the gun, the powder is directed past a point source corona field to electrostatically charge the particles. After charging, the particle laden airstream is expelled through a nozzle at a relatively high rate of speed. Generally, the powder mass flow rate from the spray system needs to be high to propel particles over a significant area of the tow. As the expelled particles near the surface of the tow, an electrostatic attraction between the charged particles and the tow causes some of the particles to adhere to the tow fibers. Unfortunately, the high powder velocity needed to effectively distribute particles over the tow often inadvertently removes previously deposited powder, which limits powder coating thickness and uniformity. Also, the momentum of rapidly moving particles may overcome electrostatic attraction to the grounded fibers, causing the particles to miss the tow.
One attempt to solve these problems might be to increase the strength of the electric field by moving the gun closer to the tow. However, the closer the gun is to the tow, the more severe the airstream removal. Powder flow through a nozzle creates a fan-shaped pattern emanating from the nozzle exit. Moving the gun closer to the tow often results in a narrower powder distribution with more difficulty coating the tow. Recognizing these limitations, one focus has been to find an optimal combination of spray gun parameters for a given resin/fiber configuration. Such parameters may include the distance from the tow, the discharge velocity, the discharge spray pattern, and electrostatic charging voltage. Even with optimization, electrostatic spray gun deposition still fails to provide the coating thickness and uniformity sought for many fiber-reinforced resin applications. Moreover, the process remains relatively slow and uneconomical for many products that would otherwise potentially benefit from the availability of less expensive composites. Furthermore, coating multiple tows generally requires multiple guns, complicating the deposition system and increasing the difficulty of obtaining a uniform deposition.
Another limitation of many powder deposition systems is that it is often difficult to uniformly impregnate the fibers with the resin after deposition. The presence of voids in the fiber/resin matrix generally weakens the composite structure and undermines reliance on various standard parameters such as resin volume fraction and fiber mass fraction of the composite. In one attempt to improve impregnation, the deposited particles may be melted and the tow pulled through a die that tapers to a fixed cross-section to "squeeze out" voids, but the pulling speed is usually quite limited to avoid damaging tow fibers. Also, a fixed die generally needs to be custom machined and hardened to withstand abrasion from the fibers, which makes it expensive. In addition, the entire mass of the die is heated, consuming significant energy.
Thus, a need remains for techniques to improve fiber tow composite manufacture. The present invention satisfies this need and provides other significant advantages.